Generally, twist drills tend to oscillate in the direction of torsion during machining. When the drill is in operation, the shank oscillates to and fro in the direction of torsion, whereby the axial position of the cutting edges is alternately moved forward and backward relative to the workpiece. Thus, the effective drill length is alternately extended and shortened during the oscillation. This cycle creates a regenerative effect in the form of a feed-back vibration phenomenon, which briefly may be described in the following way.
When a drill shank oscillates in the above-indicated way, the individual cutting edge cuts a wavy surface in the bottom of the bore hole, as is generally illustrated in FIGS. 3 and 4. Thereafter, when either one and the same cutting edge or a subsequent cutting edge (depending on whether the tool includes one or more cutting edges which cut in the same path) runs across this wavy surface, a secondary wavy surface is created (compare the continuous and dashed wave lines, respectively, in FIG. 4). In practice, this means that the nominal chip thickness will vary depending on the phase position or phase displacement between the wavy surfaces in the bottom of the bore hole. When this nominal chip thickness varies, the cutting force will also vary. This constitutes the driving force behind the above-mentioned vibration mechanism. If the chip width is increased, due to the fact that the diameter of the drill is increased at the same time as the other properties of the drill remain intact beyond a certain value, the mechanical system is no longer capable of resisting, and therefore the oscillating amplitude will grow and become unstable. By "mechanical system" is here meant primarily the helically cut drill as such. This mechanical system is primarily effected by the length of the drill shank. More precisely, a longer drill becomes more unstable in the torsion direction than a shorter one.
If the regenerative effect is triggered off, i.e. the amplitude of the vibrations grows, after a short while the vibration level will be very high. However, the growth of the amplitude decreases after a time as a consequence of other mechanisms, for instance the fact that the cutting edge cannot cut itself free.